You know you're the luckiest person alive to have landed your
perfect partner but when it comes to their relations, the laws of
probability are not on your side. You've upset the balance of their
family: at best you've swelled the ranks, further dividing the pot
of love and attention, at worst you're the cuckoo in the family
nest, stealing their beloved son/daughter/brother/sister.
Even wonderful in-laws can create problems. If they're warm, uncritical, unfailingly supportive, always pleased to see you, dedicated and hands-on with their grandchildren and generous at Christmas-time, this will play merry hell with your relationship with your own less-than-perfect parents or siblings.
Treat a bad in-law as you would childhood chicken pox. You don't want it, you don't deserve it, you can't really do much to alleviate it but it's a necessary evil and if you stay calm and are careful not to aggravate it, you'll come out barely scarred.
Comfort yourself with the thought that your partner chose to leave the bosom of his/her family and create a new family with you - if the in-laws are truly toxic then your cleverest plan is to sit tight, behave immaculately and trust that there will be no doubt about which family is the better bet.
As for perfect in-laws, don't boast about them to your own family (that way lies perdition) instead just secretly enjoy the fact you've bucked the trend.